Microbial 11.alpha.-hydroxylation of steroids is a well known process, in vivo as well as in vitro. For instance 11.alpha.-hydroxylation of progesterone by cell-free preparations of Aspergillus ochraceus has been reported by Shibahara et al., Biochim. Biophys. Acta, 202 (1970), 172-179. It has also been known that microbial 11.alpha.-hydroxylation reactions of steroids are unpredictable, and invariably lead to incomplete transformations. Typically conversion degrees of 80-85% are obtained. For industrial applications it is however of importance to obtain high predictable conversion rates, which preferably lead to higher than 95% yields of 11.alpha.-hydroxylated steroids. Mathematical models in the optimization of such fermentation processes are discussed by Deshayes et al., Bull. Soc. Chim. Fr., (1980), II 24-34. For example, in the 11.alpha.-hydroxylation of canrenone, Deshayes disclosed that under optimum conditions better than 95% yields could be attained when Aspergillus ochraceus was used in a medium containing i.a. 10 g/l of glucose in a matrix of malt-extract and trypticase. Under these conditions up to 1.5 g/l of canrenone could be transformed, which was considered to be an improvement in the art, for instance as disclosed by Blunt et al., J. Chem. Soc., 6 (1971), 1136, who were not able to obtain more than 90% yield of 11.alpha.-hydroxylated canrenone using at the most 0.5 g/l of substrate.